Field of the Invention
The present invention relates to a semiconductor laser and an optical integrated light source including the same, and for example, relates to a distributed feedback semiconductor laser.
Background Art
In response to rapidly increasing demands for communications, a wavelength division multiplexing communication system has recently been achieved. This system multiplexes a plurality of signal lights having different wavelengths, thereby enabling high-capacity transmission with a single optical fiber.
The light source for a wavelength division multiplexing communication system is preferably a single-mode laser diode, or, LD (hereinafter, referred to as a single-mode LD), which can have a high side mode suppression ratio (SMSR) of at least 30 to 40 dB or higher. A typical example of the single-mode LD is a distributed feedback laser diode (hereinafter, referred to as DFB-LD) that determines an oscillation wavelength using a diffraction grating provided in a semiconductor chip in its longitudinal direction adjacent to an active layer.
The DFB-LD selects, in accordance with the reflectance asymmetry or reflectance phase on a cleaved end surface, any one of the two oscillation modes in the vicinity of the Bragg wavelength determined by a diffraction grating period. Then, the selected mode is set as a single-mode LD. However, due to variations in threshold current and slope efficiency, a sufficiently high single-mode yield cannot be obtained.
Under the circumstances, a λ/4 phase shift DFB-LD is used which has front and rear cleaved end surfaces covered with anti-reflection coatings and has, at the central portion of the diffraction grating, a phase shift region (phase shifter) for shifting a diffraction grating phase by π. The method using such a λ/4 phase shift DFB-LD excites only one oscillation mode that matches the Bragg wavelength in principle, leading to a high single-mode yield.
Putting a wavelength division multiplexing communication system to practical use requires a wavelength tunable light source that can cover all wavelength bands and can be manufactured at low cost. There is known a monolithic integrated type wavelength tunable light source that is configured as follows (for example, see Japanese Patent Application Laid-Open No. 2003-258368). The light source includes a plurality of λ/4 phase shift DFB-LDs integrated into an array on the same substrate, in which the output sides of the LDs are connected to an input waveguide of a multimode interference type optical multiplexing circuit (hereinafter, referred to as MMI), and the lights multiplexed by the MMI are output through an output waveguide. Hereinafter, the MMI having K (K is a natural number) inputs and L (L is a natural number) outputs is represented by K×L−MMI.
Also, research has been done on monolithically integrating a λ/4 phase shift DFB-LD and a wavelength tunable light source with a Mach Zehnder (MZ) optical modulator and an electro absorption (EA) optical modulator on the same substrate.
For the wavelength division multiplexing communication system having a transmission rate of 40 Gbps or higher for a trunk line, digital coherent communications employing optical phase modulation have recently been put to practical use. The digital coherent communications require a narrow-linewidth laser light source having a laser linewidth of 1 MHz or less, more desirably, 500 kHz or less.
Unfortunately, in a single λ/4 phase shift DFB-LD, which has both end surfaces covered with anti-reflection coatings, the reflected return light from the outside of an LD resonator is apt to enter the inside of the active layer of the LD. Thus, an optical isolator needs to be provided at the output in the front of the λ/4 phase shift DFB-LD. Also, a measure against the reflected return light from the output side in back of the DFB-LD needs to be taken, limiting the flexibility in module design. If a to-be-injected current is increased for higher output, the return light itself, caused by the residual reflection on the end surface, becomes a problem.
An optical integrated device cannot include an optical isolator, and thus, inevitably suffers from the return of the reflected return light, which comes from the butt joint interface that directly bonds the waveguides formed of different epitaxial films, the output end surface, or the like, to the inside of the LD active layer.
As described above, a single λ/4 phase shift DFB-LD and an optical integrated device including the same may suffer from the following problem. Depending on the phase of return light, return-light-induced noise may be caused so that the LD linewidth increases, or a single mode oscillation may be impeded so that the SMSR decreases to 30 dB or lower.
To solve the problem above, for example, Japanese Patent Application Laid-Open No. 63-62390 (1988) proposes that first and second Bragg reflection regions be provided to the both sides of the laser light emission region including a diffraction grating. The first and second Bragg reflection regions include successive different gratings having the same phase and always have a gain in an optical wavelength, which is not more than zero. This configuration allows part of the return light to be reflected, achieving a semiconductor laser that has less return-light-induced noise than a conventional λ/4 phase shift DFB-LD and that produces a single-mode oscillation at a probability substantially identical to a conventional probability.
According to Japanese Patent Application Laid-Open No. 63-62390 (1988), unfortunately, an optical loss in a current injection region having an amplification factor of not more than zero is large because of the long length of this region, increasing threshold current and power consumption. In addition, as described below, an effect obtained by reducing the influence of the return light is not stabilized because the light intensity distribution in the laser resonator varies depending on the phase of the reflected return light.
For a λ/4 phase shift DFB-LD having a resonator length (L) of approximately 300 μm, the product (κ×L) of the coupling coefficient (κ) of the diffraction grating and the diffraction grating length (L) of the LD is normally set to approximately 1.2. To obtain a narrow-linewidth laser light source of 500 kHz or less, L needs to be 1000 μm or more to increase κ×L. This, however, may lead to multiple modes due to the influence of the reflected return light.